How to add a binary file to document library via workflow

I have a binary file in a Documents folder which needs to be periodically updated from a copy that's available either from a URL, or a network path (UNC). I can't find any workflow actions that seem to facilitate this. My best guess is that I may be able to accomplish this using the web request action, but I really don't know.

You'd think this would be a fairly common task: grab a file from [here], and add it to this document library (overwriting if necessary).